The present invention relates to a process for the production of toluene diisocyanate, in which the crude toluenediamine obtained from the hydrogenation of dinitrotoluene is purified, and then phosgenated.
It is known that di- and poly-isocyanates are produced by phosgenation of the corresponding di- and poly-amines. In the case of distillable di- and poly-isocyanates, the phosgenation and separation of the solvent used are followed by distillation of the isocyanate. For example, toluene diisocyanate (TDI) is obtained in the form of a distilled product, whereas higher molecular weight mixtures of the polyisocyanates of the diphenylmethane series are obtained in undistilled form as bottom products.
In the working up of toluene diisocyanate (TDI), the crude product, which has largely been freed of solvent, is distilled to remove high-boiling constituents as well as low-boiling constituents that remain after the solvent has been separated off. Multi-stage distillations are thereby carried out in order to separate off low boilers and high boilers that remain in the crude product [see Industrielle Aromatenchemie, H.-G. Franck, J. Stadelhöfer, Springer-Verlag Berlin, 1987, p. 253], or alternatively, more energy-efficient integrated partition columns are used as described in U.S. Published Application 2004/118672, believed to correspond to EP-A1-1413571, or in DE-A1-1260092.
In the distillation of TDI, the markedly colored crude TDI product yields a water-clear, almost colorless end product. However, even finely distilled TDI is not completely free of color-imparting components, which occasionally leads to undesirable coloring of the TDI.
Undesirable coloring of distilled TDI product can have various causes, which require different processes for their avoidance.
For example, it is described in U.S. Pat. No. 6,900,348, believed to correspond to EP-A1-1 187 808, that the use of phosgene having a bromine content of <50 ppm in the production of isocyanates of the diphenyl-methane series can lead to a pale isocyanate on phosgenation.
Likewise, EP-A1-816 333 describes a process for reducing the color of TDI, in which the crude TDI solution is treated with hydrogen before the solvent is separated off.
In addition to treatment of the crude isocyanate product after phosgenation, treatment of the amine stream can also lead to a reduction in the color of the isocyanate produced by phosgenation. U.S. Pat. No. 5,872,278, which is believed to correspond to EP-A1-866 057, describes a process in which the amine is treated, prior to the reaction with phosgene, with solid substances containing Lewis-acidic and/or Brönstedt-acidic centres. The resulting isocyanate then has a lighter color than when the untreated amine is used.